Toshihiko Ouchi

High-resolution time-of-flight terahertz tomography using a femtosecond fiber laser

High-resolution tomographic imaging is demonstrated using a reflection-type terahertz time-domain spectroscopy. To realize a practical system for general use, a robust all-fiber laser was used as the pump light source. Broadband terahertz waves were generated with the combination of optical pulses compressed to 17 fs using optical fibers and a DAST crystal. Using deconvolution signal processing, the wideband spectrum of the generated terahertz waves provided high-axial resolution leading to successful imaging of a multilayered structure containing a 2-microm-thin GaAs layer. To our knowledge, this is the first demonstration of terahertz tomographic imaging of such a thin layer.

Oscillations up to 1.40 THz from Resonant-Tunneling-Diode-Based Oscillators with Integrated Patch Antennas

Oscillations from 1.02 to 1.40 THz were observed at room temperature from resonant-tunneling-diode (RTD)-based oscillators with integrated patch antennas by improving the mesa structure and tuning the length of the antenna and the feed point location of the RTD. A two-step post structure realized a reduction of series resistance of the RTD, and the oscillation frequency significantly increased with the feed point closer to the center of patch antenna at around 1 THz. Experimental results agreed well with the analysis.

Direct coupling of VCSELs to plastic optical fibers using guide holes patterned in a thick photoresist

In this letter, we report a simple passive alignment method to couple plastic optical fibers (POFs) to vertical-cavity surface-emitting lasers (VCSELs) for use in parallel optical interconnects. Polymer guide holes were fabricated directly on the top of VCSELs by means of standard photolithography using SU-8 epoxy-based thick photoresist. Perfluorinated graded-index POFs were inserted and fixed in the guide holes to provide butt-coupled packaging modules. For a VCSEL of 15-/spl mu/m diameter and a POF of 124-/spl mu/m core diameter, coupling efficiencies were obtained between about 80% and 50%, depending on drive current. The standard deviation of the efficiency at a drive current of 10 mA was 5.9% for a 1 /spl times/ 5 array.

Terahertz spectroscopy of native-conformation and thermally denatured bovine serum albumin (BSA).

Proteins are expected to exhibit collective vibrational modes at terahertz frequencies. We have developed a promising approach to measure these motions by using a membrane device to hold samples. Samples of bovine serum albumin (BSA) in native and thermally denatured conformations were measured using terahertz time-domain spectroscopy. Clear differences were observed in transmittance and phase between native-conformation BSA samples and thermally denatured BSA samples. Time-domain data shows that samples exhibited relative time shifts when compared with a standard. Results suggest that there were differences in dielectric responses in the BSA samples, and these are probably associated with molecular conformational changes in the membrane device.

Subterahertz oscillations from triple-barrier resonant tunneling diodes with integrated patch antennas

We report on subterahertz oscillations of resonant tunneling diode oscillators with an integrated patch antenna. The device consists of an InGaAs/InAlAs triple-barrier resonant tunneling diode and a patch antenna on an InP substrate. Current-voltage characteristics indicate a peak current density of 280 kA/cm2, with a peak-valley ratio of about 3 at room temperature. An oscillation frequency of 517 GHz was observed at room temperature through Fourier transformed infrared spectroscopy. The results agree with electromagnetic analysis.

Micro Strip Line-Based On-Chip Terahertz Integrated Devices for High Sensitivity Biosensors

We have achieved label free DNA hybridization detection at the level of a few femto-moles using on-chip terahertz (THz) integrated devices. The devices consist of two photoconductive switches for the generation and detection of THz pulses. A DNA specimen placed in a reservoir on the micro strip line (MSL) connecting the switches delays the propagating THz pulses. Different delay times were observed depending on the DNA conformation. The high sensitivity achieved is due to MSL design adjustment and the use of a specimen reservoir. Both simulations and experimental results indicate that devices with a narrower MSL have higher sensitivity.

Broadband terahertz wave generation from a MgO:LiNbO 3 ridge waveguide pumped by a 1.5 μm femtosecond fiber laser

Cherenkov phase-matched terahertz (THz) wave generation from a MgO:LiNbO3 ridge waveguide was studied using optical rectification. Pumping was achieved using 20 and 60 fs laser pulses from a fiber laser centered at 1.56 μm. Time-domain spectroscopy (TDS) results showed a single-cycle pulse with 20 fs pulse pumping and a near-single-cycle pulse with 60 fs pulse pumping. The spectrum covered the range of 0.1-7 THz, with a signal-to-noise ratio of over 50 dB. The output power measured by a Si bolometer and a deuterated triglycine sulfate pyroelectric detector is shown and compared to that of a commercial photoconductive antenna. This system is believed to be a promising THz source for low-cost, compact, robust, and highly integrated TDS, THz imaging, and tomography systems.

Brain tumor imaging of rat fresh tissue using terahertz spectroscopy

Tumor imaging by terahertz spectroscopy of fresh tissue without dye is demonstrated using samples from a rat glioma model. The complex refractive index spectrum obtained by a reflection terahertz time-domain spectroscopy system can discriminate between normal and tumor tissues. Both the refractive index and absorption coefficient of tumor tissues are higher than those of normal tissues and can be attributed to the higher cell density and water content of the tumor region. The results of this study indicate that terahertz technology is useful for detecting brain tumor tissue.

Wide Spectrum Terahertz-Wave Generation From Nonlinear Waveguides

We have obtained efficient terahertz (THz)-wave generation from surface-emitting or waveguide propagation using Cherenkov-type radiation with nonlinear optical (NLO) crystals. This approach presented the following advantages: 1) many crystals can be used as THz-wave emitters; 2) it is not necessary to satisfy the phase-matching condition inside the crystal; and 3) THz-wave generation is not suppressed by the absorption in the crystal. We demonstrated that the THz-wave generation was enhanced 50× by suppressing the phase mismatch with a surfing configuration for bulk lithium niobate (LiNbO3) crystals. Using the prism-coupled Cherenkov phase-matching method with organic crystal 4-dimethylamino-N-metyl-4-stilbazolium tosylate, we produced tunable THz radiation within ~0.1-10 THz. There was no evidence of significant absorption in the crystal. To show the advantages of the waveguide emitter, we demonstrated THz generation using a ridge-type LiNbO3 waveguide with a thickness of 3.8 μm. We obtained THz-wave generation with an ideal temporal half-cycle pulse and a wide tuning range, which is applied to reflection tomography imaging.

Reduction of phonon resonant terahertz wave absorption in photoconductive switches using epitaxial layer transfer

A broadband terahertz time-domain spectroscopy (TDS) system for frequencies of up to 15 THz, including the phonon resonance frequency range, has been developed using a transferred thin-film photoconductive switch (PCS) detector. The thin-film PCSs, based on low-temperature-grown GaAs, were fabricated using epitaxial layer transfer onto high-resistivity Si substrates. We observed a reduction of phonon resonant absorption, including between 7 and 10 THz, in a forward radiation configuration. Numerically calculated absorption spectra show good agreement with our experimental results. This technique will provide compact, broadband TDS systems.